1. Field of the Invention
The present invention relates to a bearing system in a motor for a floppy disk drive apparatus (referred to as FDD hereinafter).
2. Description of the Prior Art
FDD apparatus are relatively simple for handling where floppy disks can readily be loaded and unloaded, and have widely been used as external memory devices in computers. As up-to-date computers have been down-sized, the FDD apparatuses are required to decrease their overall dimensions, particularly in microcomputers. Dimensionally decreased FDD apparatuses are substantially provided with small-sized motors for driving FDD units and magnetic heads.
The motor for driving a magnetic head has to operate at a given degree of accuracy even if its size is minimized. FIG. 3 is a cross sectional view showing a primary part of such a conventional motor as designed for satisfying the above dimensional requirement. Denoted by 51 in FIG. 3 is a side plate of an FDD apparatus. A PM type pulse motor 52 is fixedly mounted to the FDD side plate 51 for driving an unshown magnetic head carriage. More specifically, a mounting plate 53 of the pulse motor 52 is tightened by screws 54 to the FDD side plate 51. The pulse motor 52 has a stator 55 and a rotor 56 with permanent magnets mounted on the inner side of the stator 55 for rotation. The construction of the PM type pulse motor 52 is well known and will be explained in no more details, except in connection with a bearing structure.
The rotor 56 is mounted to a rotary shaft 57 which extends to the outside of the pulse motor 52 having a lead-screw portion 58 thereof for movement of the magnetic head carriage. The stator 55 of the PM type pulse motor 52 has a protective cap 59 of a metal sheet provided at one end thereof. A thrust spring 60 of a leaf type is sandwiched between the stator 55 and the protective cap 59 for urging the rotary shaft 57 in the thrust direction. The rotary shaft 57 is rotatably supported at one end by a ball 61 disposed at the distal end of the thrust spring 60. There is a sintered metal bearing 62 mounted to the mounting plate 53 on the side wall 51 in order to rotatably support an intermediate portion of the rotary shaft 57. The rotary shaft 57 is also supported at the other end by a ball 64 seated on a pivot bearing 63 which is fixedly mounted to a bottom plate (not shown) of the FDD apparatus. The ball 61 is coated with a later of wear-protective grease.
In such a conventional bearing system in an FDD motor as explained above, the rotary shaft 57 is substantially supported by the sintered metal bearing 62 and the pivot bearing 63. Hence, the rotational accuracy of the rotary shaft 57 is determined by a combination of the surface quality of the FDD bottom plate and the installation precision of the pivot bearing 63 on the bottom plate, and the surface quality of the mounting plate 53 of the PM type pulse motor 52 and the installation precision of the sintered metal bearing 62 on the mounting plate 53. The combination may thus affect the operational accuracy of the leadscrew portion 58 of the rotary shaft 57, causing write and read actions of the FDD apparatus to be unstable.